The Islamic State extremist group has taken control of a vast former chemical weapons facility northwest of Baghdad, where remnants of 2,500 degraded chemical rockets filled decades ago with the deadly nerve agent sarin are stored along with other chemical warfare agents, Iraq said in a letter circulated Tuesday at the United Nations.

The U.S. government played down the threat from the takeover, saying there are no intact chemical weapons and it would be very difficult, if not impossible, to use the material for military purposes.

Iraq's UN Ambassador Mohamed Ali Alhakim said in a letter to UN Secretary-General Ban Ki-moon in a letter circulated Tuesday that "armed terrorist groups" entered the Muthanna site on June 11, detained officers and soldiers from the protection force guarding the facilities and seized their weapons. The following morning the project manager spotted the looting of some equipment through the camera surveillance system before the "terrorists" disabled it, he said.

The Islamic State in Iraq and Syria (ISIS), which controls parts of Syria, sent its fighters into neighbouring Iraq last month and quickly captured a vast stretch of territory straddling the border between the two countries. Last week, its leader, Abu Bakr al-Baghdadi, declared the establishment of an Islamic state, or caliphate, in the land the extremists control.

Alhakim said as a result of the takeover of Muthanna, Iraq is unable "to fulfill its obligations to destroy chemical weapons" because of the deteriorating security situation. He said it would resume its obligations "as soon as the security situation has improved and control of the facility has been regained."

Alhakim singled out the capture of bunkers 13 and 41 in the sprawling complex 56 kilometres northwest of Baghdad in the notorious "Sunni Triangle."

The last major report by UN inspectors on the status of Iraq's weapons of mass destruction program was released about a year after the experts left in March 2003. It states that Bunker 13 contained 2,500 sarin-filled 122-mm chemical rockets produced and filled before 1991, and about 163 tonnes of sodium cyanide, "a very toxic chemical and a precursor for the warfare agent tabun."

The UN said the bunker was bombed during the first Gulf War in February 1991, which routed Iraq from Kuwait, and the rockets were "partially destroyed or damaged."

It said the sarin munitions were "of poor quality" and "would largely be degraded after years of storage under the conditions existing there." It said the tabun-filled containers were all treated with decontamination solution and likely no longer contain any agent, but "the residue of this decontamination would contain cyanides, which would still be a hazard."

According to the report, Bunker 41 contained 2,000 empty 155-mm artillery shells contaminated with the chemical warfare agent mustard, 605 one-tonne mustard containers with residues, and heavily contaminated construction material. It said the shells could contain mustard residues which can't be used for chemical warfare but "remain highly toxic."

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